1. Field of the Invention
The present invention relates generally to systems and methods for determining characteristics of a formation surrounding a borehole. More particularly, the present invention relates to an acoustic logging tool that provides enhanced performance through use of a quadrupole source configuration.
2. Description of Related Art
An acoustic logging tool typically includes an acoustic source (transmitter), and a set of receivers that are spaced several inches or feet apart. An acoustic signal is transmitted by the acoustic source and received at the receivers of the borehole tool which are spaced apart from the acoustic source. Measurements are repeated every few inches as the tool passes along the borehole.
The acoustic signal from source travels through the formation adjacent the borehole to the receiver array, and the arrival times and perhaps other characteristics of the receiver responses are recorded. Typically, compressional wave (P-wave), shear wave (S-wave), and Stoneley wave arrivals and waveforms are detected by the receivers and are processed. The processing of the data is often accomplished uphole or may be processed real time in the tool itself. Regardless, the information that is recorded is typically used to find formation characteristics such as formation slowness (the inverse of acoustic speed), from which pore pressure, porosity, and other formation property determinations can be made. In some tools, the acoustic signals may even be used to image the formation.
Logging-while-drilling (LWD) logging tools are generally located as close to the drill bit as possible, so as to minimize the delay between reaching a formation and measuring its properties. When implemented as LWD tools, acoustic logging tools must overcome a number of obstacles to perform successfully. These obstacles include drilling noise, and acoustic properties of the thick tool body. The drilling operation creates a continuous low-frequency noise that can interfere with acoustic measurements. For the most part, the noise of the drilling operation exists only in a range of frequencies below about 2 kHz. Accordingly, the drilling noise can be effectively screened out in tools designed to operate in higher frequency ranges.
The portion of the drill string near the drill bit is typically designed to withstand the large compressive force created by the weight of the drill string resting on the drill bit. Accordingly, the walls of the tubing in this region is substantially thicker than the average wall thickness of the drill string. The outer-diameter of the tubing is consequently increased, particularly for LWD tools, which have to accommodate the bulk of logging instruments in addition to an inner bore for fluid flow.
In acoustic tools, increasing the rigidity of the tool increases the amplitude of “the tool mode”, i.e. the amplitude of the acoustic energy that propagates through the body of the tool between the acoustic source and the receiver array. It is desirable to minimize the tool mode because this energy can interfere with the desired measurements of the true formation shear wave velocity.
Increasing the outer diameter of the tool (relative to the borehole diameter) also increases the dispersion of interface waves. Interface waves are acoustic energy that propagates along the borehole boundary. For soft formations where the shear wave velocity is slower than the borehole fluid sound velocity, the interface wave velocity provides the best measurable indication of the true formation velocity. Dispersion of these waves reduces the measurement accuracy. Accordingly, it is desirable to reduce the dispersion of interface waves.